1. Field of the Invention
This invention relates generally to printed wiring boards and, more particularly, to a technique and the resultant product for forming printed wiring boards wherein the technique includes laminating at least two layers together to form the printed wiring board.
2. Background of the Invention
High frequency applications in the field of printed wiring boards, i.e. one GHz and above, are driving the need for smooth copper features on the surfaces of the signal lines. This need is due to the so-called skin effect where, as frequency increases, the path of the electrical signal tends toward the outer surface of the conductor. Hence, roughness on the surface of the copper in high frequency applications will result in higher surface resistivity and longer effective line length, both of which contribute to higher conductive losses for the signal. Also, signal integrity can be affected by the roughness in the ground or voltage planes that are referenced by a signal line in a composite board structure.
However, conventional printed wiring board processes, such as lamination, depend on roughened copper surfaces in order to provide adequate adhesion of the copper to dielectric laminate in the composite laminated structure. Typically, the exposed surfaces of internal wiring planes and voltage planes of a layer prior to lamination are initially smooth, and then are roughened to promote adhesion. Techniques for roughening the copper include the oxide and oxide replacement processes, as well as the application of brass, or zinc and/or nickel on the copper surface. Conventionally, the roughening treatment is applied to all the exposed copper surfaces prior to, as well as after, personalization of the copper plane. Thus, the two competing problems require different surface roughnesses for optimum benefit; i.e., a very smooth surface of the conductive material is desired for a most efficient signal propagation, while a roughened surface is desired for optimum adhesion of copper to the dielectric material.